(i) Field of the Invention
The present invention relates to a liquid composition useful in the cold preparation of composite materials to confer to them without preliminary sanding or abrasion a surface state favorable for adhesive bonding with the aid of polyurethane type adhesives.
(ii) Description of the Related Art
Composite materials principally comprise two components: an unsaturated polyester-type resin and at least one reinforcing filler comprised of glass fibers.
These composite materials are most often found as either mass composites or laminated composites.
The mass composite materials principally comprise three components: 1) an unsaturated polyester resin (UP) (about 30% by weight), 2) a reinforcing filler generally comprised of glass fibers (about 20% by weight), and 3) an inert packing filler (about 50% by weight) often comprised of calcium carbonate, which is intended to reduce the cost of the material. Naturally, other additives can be added. For example, thermoplastic additives such as polyvinyl acetate (PVAc) can be added, which can both limit the shrinkage of the resin during the polymerization and promote smooth surfaces. Internal mold-release agents such as zinc or calcium stearates can also be added to facilitate the release of the composite materials from a mold.
When these composite materials are molded when cold and/or under a vacuum, they are called contact polyesters. When molded by heat, they are referred to by a series of abbreviations such as SMC (Sheet Molding Compound), BMC (Bulk Molding Compound), RTM (Resin Transfer Molding), AMC (Adapted Molding Compound), and RIM (Resin Injection Molding), etc.
The automobile industry is representative of the users of this type of composite material. This industry has been increasingly using these materials to replace sheet metal, thereby reducing weight, resolving corrosion problems, and enabling new forms. The joining of such materials by adhesive bonding is the only means of available for assembly because classic techniques such as fastening and bolting are not suitable. Moreover, after painting, the finished surface must present a smooth appearance identical to that of the sheet metal that also may be present on the vehicle.
The terms GRP (Glass Reinforced Plastics) or FRP (Fiber Reinforced Polymers) designate any laminated composite material or composite piece including an unsaturated polyester type resin matrix and reinforcement fibers, including glass fibers. In the manufacture of a GRP piece, lamination begins after a “gel coating” is applied and is in its hardening phase. Lamination is achieved by creating a series of successive reinforcing resin layers on the gel coat that after debulking results in a hard and compact structural assembly of the pieces.
The reinforcement material may comprise a glass mat, cut glass threads or glass tissues, or other materials that impart optimal structural properties in the molded object. Fibers based on synthetic polymers (in roving, mat, or non-woven cloth form), inorganic materials or compounds such as boron, carbon (graphite) or silica (in roving or mat form) are also suitable. Glass fiber reinforcements such as the following are frequently used and are known to those of skill in the art:                glass mat or web        cut fiber mat        Roving or Stratifil,        Roving tissue        fabric or tissue        sandwich materials        
For a lamination resin, the choice of reinforcement is made according the mechanical characteristics desired, including the number of layers, the thickness of the laminate, and its amount of glass.
The resin can be manually applied, by brush, by spraying, or by injection. The technique chosen varies according to the form, size, and number of pieces to be produced.
Polyester resins are widely used and yield excellent results for contact molding. For applications requiring specific mechanical properties, vinyl-ester resins and epoxy resins are also used. Whatever type of resin is used, the cold transformation is an irreversible change for the molded resin from a liquid state to a solid state, during which, by chemical reaction, the material acquires its final characteristics. This transformation is therefore an important stage which not only governs the behavior and the properties of the molded piece but also its surface state.
The leisure boating industry is representative of the users of the contact molding technique. This open mold lamination process is carried out manually or by simultaneous spraying of the glass and of the resin, which is called low pressure lamination. The leisure boating industry frequently uses unsaturated polyester-resin-based laminates to make boat hulls, which incorporate, before the fitting of the deck, a number of bulkheads that are positioned according to the desired boat model. In such an application, the use of adhesive bonding is the only available means of assembly because classical techniques such as fastening and bolting are not suitable.
A polyurethane is generally used and may comprise the mono-component (PU1K or HMPUR) or bi-component (PU2K) type.
The quality of the adhesive bond between two pieces, where at least one of the pieces is made of a composite material, is thus a necessary concern when minimizing the costs of preparation of the surfaces to bond.
The quality of the adhesive bond is determined by tensile and shearing strength rupture resistance tests according to the “hold method” which defines breaking patterns. It is necessary to distinguish the adhesive rupture (AR), considered to be bad, when the adhesive remains integral with a support, the cohesive rupture (CR) considered to be good, where the rupture takes place in the middle of the adhesive layer, and the support rupture (SR), considered to be excellent, where the support ruptures independent of the adhesive, known as “support delamination” (DEL). CR and SR involve “structural or semi-structural” type rupture. A series of intermediate cases typically exist which are assessed by percentage of the affected surfaces.
In the case of a laminated composite, the laminate represents the largest part of any GRP piece, and is therefore what confers the mechanical resistance to the molded piece. This requires the elimination of the zones that are poor or rich in resins by a uniform distribution of the resin and the reinforcement and a rigorous control of the resin/glass ratio. Finally, the number of air cavities or of small surface craters is eliminated with a careful “debulking” which gives a vitrified appearance to the piece. The very closed polyester structure has an influence on the surface characteristics of the composite and consequently on its adhesive behavior.
A solvent such as methyl ethyl ketone, dichloromethane, and/or acetone is typically used to treat the surfaces of the pieces to be adhesively bonded. These solvents act as degreasing agents for the partial solubilization and/or the crude elimination of the internal mold release agents and of the thermoplastic phase present at the surface. This treatment is not however sufficiently effective to avoid a preliminary sanding or abrasion of the pieces to be bonded before the “degreasing,” and this mechanical sanding or abrasion operation is a source of unwanted dust. The sanding or abrasion is also made more difficult by complex geometries of the pieces to be bonded.
Chemical treatments of the substrate adapted to the adhesives used are also proposed.
U.S. Pat. No. 5,792,203 describes the use of amine based compositions in a non-volatile fluid solvent in order to promote the adhesive bonding of plastics by means of cyanoacrylate-based adhesive compositions.
U.S. Pat. No. 4,397,707 describes an adhesive bonding process of polyester-based materials by means of a reactive isocyanate/aminoresin coating (urea formaldehyde or melamine formaldehyde).